The present invention relates generally to chemical mechanical polishing (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to carrier heads for wafers and pad conditioning pucks, in which repeatability is provided in measuring forces applied to the heads eccentrically of a main axis of the head are resisted, wherein the heads, with the wafers and the pucks, do not tilt in response to the eccentric forces, but instead the heads are allowed to move parallel to a wafer axis; and relates to facilities for CMP operations, such as facilities for supplying fluids to, and removing fluids from, the carrier heads for the CMP operations without interfering with the CMP operations.
In the fabrication of semiconductor devices, there is a need to perform CMP operations, including polishing, buffing and wafer cleaning. For example, a typical semiconductor wafer may be made from silicon and may be a disk that is 200 mm or 300 mm in diameter. For ease of description, the term xe2x80x9cwaferxe2x80x9d is used below to describe and include such semiconductor wafers and other planar structures, or substrates, that are used to support electrical or electronic circuits.
Typically, integrated circuit devices are in the form of multi-level structures fabricated on such wafers. At the wafer level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. According to the type of CMP operation being performed, certain materials, such as slurry, are used to facilitate and enhance the CMP operation. For example, the slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
In a typical CMP system, a wafer is mounted on a carrier with a surface of the wafer exposed. The carrier and the wafer rotate in a direction of rotation. The CMP process may be achieved, for example, when the exposed surface of the rotating wafer and a polishing pad are urged toward each other by a force, and when the exposed surface and the polishing pad move or rotate in a polishing pad direction. Some CMP processes require that a significant force be used at the time the rotating wafer is being polished by the polishing pad.
Normally, the polishing pads used in the CMP systems are composed of porous or fibrous materials. However, in some CMP systems, the polishing pads may contain fixed abrasive particles throughout their surfaces. Depending on the form of the polishing pad used, the slurry may be composed of an aqueous solution such as NH4OH, or DI water containing dispersed abrasive particles may be applied to the polishing pad, thereby creating an abrasive chemical solution between the polishing pad and the exposed surface of the wafer.
Several problems may be encountered while using a typical CMP system. One recurring problem is called xe2x80x9cedge-effect,xe2x80x9d which is caused when the CMP system polishes an edge of the wafer at a different rate than other regions of the wafer. The edge-effect is characterized by a non-uniform profile on the exposed surface of the wafer. The problems associated with edge-effect can be divided to two distinct categories. The first category relates to the so-called xe2x80x9cpad rebound effectxe2x80x9d resulting from the initial contact of the polishing pad with the edge of the wafer. When the polishing pad initially contacts the edge of the wafer, the pad rebounds (or bounces off) the edge, such that the pad may assume a wave-like shape. The wave-like shape may produce non-uniform profiles on the exposed surface of the wafer.
The second category is the xe2x80x9cbum-offxe2x80x9d effect. The bum-off effect occurs when a sharper edge of the wafer is excessively polished as it makes contact with the surface of the polishing pad. This happens because a considerable amount of pressure is exerted on the edge of the wafer as a result of the surface of the pad applying the force on a very small contact area of the exposed surface of the wafer (defined as the edge contact zone. As a consequence of the burn-off effect, the edges of the resulting polished wafers exhibit a burn ring that renders the edge region unusable for fabricating silicon devices.
Another shortcoming of conventional CMP systems is an inability to polish the surface of the wafer along a desired finishing layer profile. Ordinarily, the exposed surface of a wafer that has undergone some fabrication tends to be of a different thickness in the center region and varies in thickness out to the edge. In a typical conventional CMP system, the pad surface covers the entire exposed surface of the wafer. Such pad surface is designed to apply a force on a so-called xe2x80x9cfinishing layerxe2x80x9d portion of the exposed surface of the wafer. As a result, all the regions of the finishing layer are polished until the finishing layer is substantially flat. Thus, the surface of the pad polishes the finishing layer irrespective of the wavy profile of the finishing layer, thereby causing the thickness of the finishing layer to be non-uniform. Some circuit fabrication applications require that a certain thickness of material be maintained in order to build a working device. For instance, if the finishing layer were a dielectric layer, a certain thickness would be needed in order to define metal lines and conductive vias therein.
These problems of prior CMP operations, and an unsolved need in the CMP art for a CMP system that enables precision and controlled polishing of specifically targeted wafer surface regions, while substantially eliminating damaging edge-effects, pad rebound effects, and edge bum-off effects, are discussed in related United States patent application Ser. No. 09/644,135 filed Aug. 22, 2000 for Subaperture Chemical Mechanical Polishing System and assigned to the assignee of the present application (the xe2x80x9crelated applicationxe2x80x9d). The specification, claims and drawings of such related application are by this reference incorporated in the present application.
In such related application, a CMP system follows the topography of layer surfaces of the exposed surface of the wafer so as to create a CMP-processed layer surface which has a uniform thickness throughout. Such CMP system implements a rotating carrier in a subaperture polishing configuration, eliminating the above-mentioned drawbacks, edge-effects, pad rebound effects, and edge bum-off effects. For example, one embodiment of such CMP system includes a carrier having a top surface and a bottom region. The top surface of the carrier is designed to hold and rotate a wafer having one or more formed layers to be prepared. Further included is a preparation head, such as a polishing head, designed to be applied to at least a portion of the wafer, wherein the portion is less than an entire portion of the surface of the wafer. Although such CMP system avoids the above-described edge-effects, pad rebound effects, and edge bum-off effects, the application of such preparation head in this manner applies a force to the exposed surface of the wafer and to the carrier at a location that is eccentric with respect to an initial orientation of the wafer and the carrier. The initial orientation includes an initial orientation of central axes of the wafer and of the carrier (which are coaxial and positioned substantially vertically). The initial orientation also includes an initial orientation of the exposed surface of the wafer (which is positioned at an initial angle of ninety degrees with respect to the initial substantially vertical orientation of the central axes of the wafer and the carrier). The term xe2x80x9csubstantially verticalxe2x80x9d means true vertical, and includes true vertical plus or minus normal mechanical tolerances from true vertical, such as those tolerances typical in bearings used in spindles and other supports for such carriers.
As may be understood from the above discussion of the edge-effects, pad rebound effects, and edge bum-off effects, it would be undesirable for such eccentric force to cause the central axes of the wafer and the carrier to depart from the initial orientation and to tilt, or assume a tilted orientation, under the action of the eccentric force. Such tilting or tilted orientation would occur when such central axes of the wafer and/or the carrier depart from true vertical more than the above-described normal mechanical tolerances from true vertical, e.g., by a number of degrees. In the prior art, gimbals are used as supports for carriers that present wafers to a preparation head, such as a head having a polishing pad, for example. The gimbals allow the wafer carrier (with the wafer mounted thereon) to tilt and assume such a tilted orientation relative to such initial orientation of the central axes of the wafer and the carrier. As described above- such tilting allows the exposed surface of the wafer to be at an angle other than substantially vertical, such as about eighty-five to eighty-eight degrees from horizontal, which is a significant departure from the initial orientation described above. Thus, due to the allowed tilting, the exposed surface of the wafer is not perpendicular to the initial orientation of such central axis of the wafer and the carrier. The tilting allowed by such gimbals may be appropriate when the polishing pad has an area about the same as that of the exposed surface of the wafer and the area of the pad totally overlaps the area of the exposed surface of the wafer. However, in the eccentric force situation described above (i.e., when the area of the polishing pad, for example, does not totally overlap the area of the exposed surface of the wafer) such gimbals, may not be used. In detail, such initial orientation of the central axes of the wafer and the wafer carrier is the orientation that must be maintained during polishing under the action of such eccentric force to achieve the desired planarization of the exposed surface of the wafer. In other words, the tilting allowed by such gimbals must be avoided if the desired planarization of the exposed surface of the wafer is to be achieved.
In U.S. Pat. No. 4,244,775, a polishing plate is provided with a diameter about twice that of a semiconductor body to be treated. The body is mounted in a supporting holder in a manner that presents an entire surface of the body to the polishing plate. As a result, movement of the body and of the support holder within a collar toward and away from the polishing plate always presents the entire surface of the body to the polishing plate. Because the support holder surrounds the body, the holder must have a relatively large diameter, e.g., more than eight inches if the semiconductor body is an eight inch diameter wafer. Thus, in the example of such wafer, the length of the collar (which would generally be twice the diameter) would be about sixteen inches. As a result of this configuration of the collar relative to the semiconductor body, the length of the collar is directly related to the diameter of the semiconductor body to be processed. Further, with such large collar, frictional losses would be relatively large between the collar and the support holder, and may be variable as well.
In addition, in the past wafer carriers have been provided with flat metal backings on which the wafer is directly placed. One such wafer carrier provides a number of holes through the metal backing by which a vacuum is applied to the wafer. In theory, a wafer present on the metal backing will block the flow of air into the holes, changing the pressure in a duct to the holes, providing a way to indicate the presence of the wafer. However, vacuum applied through such holes can deform the wafer and interfere with the accuracy of polishing operations on the wafer on the metal backing. Also, slurry used in the polishing operations can block one or more of the holes, and result in a false indication of wafer presence on the metal backing.
Another type of wafer carrier provides a ceramic layer on the carrier. Such layer has one-half micron to one micron pores. Investigation relating to the present invention indicates that such extremely small micron-size pores could easily clog and would be difficult to clear. Generally, such carriers are cleaned by fluid sprayed onto the top of the carrier on which a wafer is placed, for example. Thus, such sprays are applied externally of such ceramic layers even though the clogged, very small micron-size pores are inside the layer.
Also, in another type of polishing system, the exposed surface of a wafer to be polished, for example, faces downwardly, and may be horizontal. In this type of system, slurry used for polishing more easily flow off, or be removed from, the exposed surface and parts of the carrier. As a result, this type of system does not present the problem of removal of slurry from an exposed surface that faces upwardly.
Another problem faced in providing preparation heads, such as wafer polishing heads, is that one head may be used to carry a particular wafer during many different processing steps (e.g., wafer polishing and buffing) Here, the carrier with the wafer attached, is first mounted at one processing station, and processed. Upon completion of the first processing, the carrier is removed from the first station, transported to a second station, and mounted at the second processing station, etc. As a result, currently there are significant demands for very small carriers that may be universally used with many type of processing stations.
What is needed then, is a CMP system and method in which a force applied to a carrier, such as a wafer or puck carrier, may be accurately measured even though such force is eccentrically applied to such carrier. In particular, currently there is an unmet need for a way of providing an accurate indication of an amount of such eccentric force. Such an accurate indication is a repeatable measurement technique that may be described in terms of xe2x80x9cequal eccentric forcesxe2x80x9d. Such equal eccentric forces are eccentric forces having the same value as applied by a pad, such as a polishing pad, to a carrier for a wafer or pad conditioner puck. The repeatable measurement technique is one which, for all such equal eccentric forces, the loss of force within the measurement system and within the system for supporting the carrier, will be substantially the same, i.e., repeatable. Moreover, what is needed is a CMP system and method having the above-described needed repeatable measurement features, while providing facilities for other CMP operations, such as facilities for supplying fluids within a carrier to the wafer and a wafer support without interfering with the polishing operations. Similarly, what is needed is a CMP system and method for removing fluids from, the carrier for the CMP operations without interfering with the CMP operations.
Broadly speaking, the present invention fills these needs by providing CMP systems and methods which implement solutions to the above-described problems, wherein structure and operations are provided that facilitate making repeatable measurements of the eccentric forces. In such systems and methods, a force applied to a carrier, such as a wafer or puck carrier, may be accurately measured even though such force is eccentrically applied to such carrier. Another aspect of such systems and methods of the present invention is a CMP system and method having the above-described needed repeatable measurement features, while providing facilities supplying fluids within a carrier to the wafer and a wafer support without interfering with the polishing operations. Similarly, another aspect of such systems and methods of the present invention is a CMP system and method for removing fluids from the wafer or puck carrier without interfering with the CMP operations.
In one embodiment of the systems and methods of the present invention, an initial coaxial relationship between an axis of rotation and a carrier axis is maintained during application of the eccentric force, such that a sensor is enabled to make repeatable measurements, as defined above, of the eccentric forces, and the carrier may be a wafer or a puck carrier.
In another embodiment of the systems and methods of the present invention, such initial coaxial relationship is maintained by a linear bearing assembly mounted between the carrier and the sensor, and the carrier may be a wafer or a puck carrier.
In yet another embodiment of the systems and methods of the present invention, the linear bearing assembly is provided as an array of separate linear bearing assemblies, wherein each separate linear bearing assembly is dimensioned independently of the diameter, for example, of a wafer or puck carried by the carrier.
In still another embodiment of the systems and methods of the present invention, the linear bearing assembly is provided as an array of separate linear bearing assemblies in conjunction with a retainer ring movable relative to the carrier, wherein an eccentric force applied to the retainer ring is accurately measured even though such force is eccentrically applied to such ring.
In a related embodiment of the systems and methods of the present invention, the linear bearing assembly is assembled with the retainer ring in conjunction with a motor for moving the ring relative to the wafer mounted on the carrier so that an exposed surface of the wafer and a surface of the retainer ring to be engaged by the polishing pad are coplanar during the polishing operation.
A further embodiment of the systems and methods of the present invention provides a vacuum chuck supplied with both a vacuum and a wash fluid through the same conduit system, wherein the vacuum is applied to the wafer uniformly across the vacuum chuck and through large-micron-size pores that may easily be cleaned by wash fluid fed through the same conduit system.
Another beneficial embodiment of the systems and methods of the present invention provides a portion of the wafer overhanging the carrier, in conjunction with passageways in the carrier for directing wash fluid against the overhanging portion to clean slurry from the carrier.
An added embodiment of the systems and methods of the present invention provides a puck made from a perforated plate in which perforations extend across a surface for supporting the puck and a fluid is distributed substantially all across the puck to purge the puck.
A still additional embodiment of the systems and methods of the present invention provides a puck support having a lip defining a reservoir for receiving a puck having perforations, wherein the puck support is configured to distribute fluid to all of the perforations to fill the reservoir.